The present invention relates to the field of fiber optic communications and in particular, to a method for calibrating a micro-electromechanical (MEMS) device.
In fiber optic communication systems, signal routing is the ability to direct a signal received from one of a plurality of input fibers or ports to any of a plurality of output fibers or ports without regard to the frequency and polarization of the optical signal. Signal routing is essential for directing an optical signal carrying data to an intended location.
Free-space optical crossconnects allow interconnecting among input and output ports in a reconfigurable switch fabric. An example of such an optical crossconnect utilizes an array of MEMS tilting mirror devices as the fabric. By adjusting the tilt angles of the MEMS mirror devices, optical signals can be directed to various destinations, i.e., to numerous output fibers.
Arrays of two-axis tilt mirrors implemented using micro-electromechanical systems (MEMS) technology allow for the construction of large scale optical crossconnects for use in optical systems. Optical crossconnects are commonly employed to connect a number of input optical paths to a number of output optical paths. A typical requirement of optical crossconnects is that any input be capable of being connected to any output. One example of a MEMS device is the MEMS mirror array 10 depicted in FIG. 1. The mirror array 10 includes a plurality of tilt mirrors 12 formed on a substrate 11, mounted to springs 14 and controlled by electrodes (not shown). Each mirror 12 is approximately 100-500 microns across, may be shaped as square circular or elliptical, and is gimbaled with the tilt angle being selectively determined by the amount of voltage applied to the control electrodes. Gimbaled mirrors are capable of operatively rotating or tilting about at least two axes, for example, orthogonal X-Y axes of rotation. With two axes, one axis is termed the mirror axis, the other axis (typically orthogonal to the mirror axis) is the gimbaled axis. Gimbaled mirror configurations are described in U.S. Pat. No. 6,201,631 to Greywall. Other mirrors, with only one axis, are also known in the art.
Further details of the operation of the MEMS mirror array 10 are found in copending U.S. patent application Ser. No. 09/415,178, filed Oct. 8, 1999. Utilizing two or more such tilt mirror arrays 10 to form an optical crossconnect is disclosed in copending U.S. patent application Ser. No. 09/410,586 filed Oct. 1, 1999. Techniques associated with monitoring mirror position are disclosed in copending U.S. patent application Ser. No. 09/414,621 filed Oct. 8, 1999. Techniques for detecting mirror position are disclosed in copending U.S. patent application Ser. No. 09/518,070 filed Mar. 3, 2000. The entire contents of each of the above-mentioned patent applications are hereby incorporated by reference.
The use of one or more MEMS tilt mirror arrays in conjunction with a lens array is disclosed in co-pending U.S. patent application Ser. No. 09/512,174, filed Feb. 24, 2000, the entire content of which is also incorporated herein by reference. As disclosed in that application, various optical crossconnect configurations of compact size (i.e. minimal spacing between crossconnect components) and exhibiting minimal optical power loss can be realized. One such optical crossconnect 100 discussed in the aforementioned application is depicted in FIG. 2. Crossconnect 100 receives input optic signals 108 through a plurality of optic fibers 112a, 112b, 112c, 112d, preferably formed in an array 112 as is well known in the art. For ease of illustration, fiber array 112 is shown as a one-dimensional array having four fibers 112a, 112b, 112c, 112d. It is in any event to be understood that fiber array 112, as well as other fiber arrays discussed herein are preferably two-dimensional arrays such as, for example, Nxc3x97N arrays.
Fiber array 112 transmits the optical signals 108 to an array of lenses 114 that function as collimating lenses. The lens array 114 is positioned relative to fiber array 112 so that each lens communicates with a corresponding fiber for producing beams 116 from the optic signals 118. Thus, beam 116a is produced from a signal carried by fiber 112a, beam 116b is produced from a signal carried by fiber 112b, etc.
A first MEMS tilt mirror array 10a, also referred to as the input array, is positioned in alignment with the lens array 114 so that each mirror element 12a will receive a corresponding beam 116. The mirror elements 12a are operatively tilted, in a manner discussed in application Ser. No. 09/415,178, to reflect the respective beams 116 to a second or output MEMS mirror array 10b positioned in optical communication with MEMS array 10a. Depending on the tilt angle of each mirror element 12a in input MEMS array 10a, the reflected signals can be selectively directed to specific mirror elements 12b in output MEMS array 10b. 
To illustrate this principle, beam 116a is shown in FIG. 2 generating reflection beams 120a and 120axe2x80x2 and beam 116d is shown generating reflection beams 120d and 120dxe2x80x2. The particular trajectory of the reflection beams is determined by the tilt angle of the mirrors in the MEMS array 10a, on which the beam 116 is incident. These beams are received by mirror elements 12b in the output MEMS array 10b and are directed as beams 124a to an output lens array 126. An output fiber array 128 is aligned with lens array 126 to receive and transmit output optical signals 129. Thus, lens array 126 couples beams 124 into the output fiber array 128.
MEMS devices 10a and 10b, and in particular, tilting mirror devices 12a and 12b, are fairly sensitive devices which may be moved by the application of a force and may require fairly precise positioning. Knowledge of the devices"" 10a and 10b response to an applied force is important to controlling the position of the mirrors 12a and 12b. Further, acquiring this knowledge as quickly as possible is also an important criterion.
The present invention is directed to a method of calibrating a MEMS device such that the response of each of the elements of the MEMS device to the applied force is known.
The present invention is directed to a method of calibrating a MEMS device such that the MEMS device is calibrated quickly and accurately.
In the present invention, various voltages are applied via electrodes to create potentials between mirrors of the MEMS device and the electrodes to move the mirrors. The potentials cause the mirrors to rotate. The relationship between the applied voltage and the mirror rotation (in angle or position) is recorded as a calibration curve. In one embodiment, this relationship is determined for every mirror in the MEMS device. Next, the trajectory of a beam reflected by a mirror is determined as a function of mirror position. Determining the trajectory of the beam determines where the beam is directed by the mirror. In an optical cross-connect the beam is directed to a location on another component (e.g. another moving mirror in a different array, a non-moving optical element, an output fiber, etc.) In one embodiment, raytracing is used to determine where a beam will be directed as a function of mirror position. The angles of the mirrors associated with directing the beam to a particular location are determined from the ray tracing. In one embodiment, this relationship is determined for every mirror in the MEMS device. The calibration curve and the raytraces provide the voltages to be applied to move mirrors in a perfectly aligned cross-connect to direct a beam to desired locations.
Once this information is obtained, the optical interconnect is actually physically assembled. A subset of the mirrors are tested to determine the voltages that are actually needed to move the mirrors to direct the beams to the desired locations. The differences between the voltages actually required to make the connection (possibly after some minor adjustments) and the voltages provided based upon the calibration curve and the raytraces are determined. These differences represent a transformation that indicates the differences between a perfectly aligned crossconnect and a crossconnect as actually physically assembled.
The transformation may be more complicated than just a scalar, numerical value or values. The transformation may be a mathematical transformation, which relates the actual experimental voltages to calculated voltages of the perfectly aligned crossconnect, and may include a set of multivariable polynomials that describe the rotation, translation, tilt and distortion. With regard to the offset, once an initial and partial set of voltages are determined, the coefficients of the polynomial mapping are calculated and applied to calculate a new set of voltages. The transformation is used to update the values represented by the calibration curve and the raytraces. New actual measurements may then be taken and compared to the updated values represented by the calibration curve and the raytraces. The updated values and the actual measured values begin to converge, after, in a preferred embodiment, two to three times. In a preferred embodiment, a different number of mirrors are actually measured in each iteration. In a more preferred embodiment, more mirrors are actually measured in each subsequent iteration (each mirror being closer to actual in subsequent iterations). In a more preferred embodiment, four mirrors are used for the first iteration and 16 for a second.
One advantage of the method of the present invention is that every mirror need not be tested or trained after the crossconnect is assembled, thereby decreasing the time needed to calibrate the crossconnect.